The present invention relates to battery chargers, and more particularly to improved control means for operating thyristors in the charging circuit thereof.
Many circuits have been developed for the purpose of charging batteries, and particularly for transferring energy from an AC power source, such as utility power lines, to a battery. In principle, devising a circuit which will transfer charge is quite straightforward; however, it is difficult to properly control the charging rate for various batteries having different characteristics and states of charge. With the advent of the thyristor, and particularly the SCR, control of current rectification and thus of battery charging current has been increased considerably. As a result, battery charging circuits using SCR's have become commonplace and a great deal of effort has been expended in devising control circuitry for SCR's.
One major problem in controlling SCR operation is the necessity of commutating the SCR. As is well known, SCR's will ordinarily not conduct current in a forward direction unless and until an appropriate current signal is applied to their gate terminals. Once the SCR begins to conduct it remains in a conducting state, even after the gating signal terminates, as long as the SCR is forward-biased and a minimum amount of current flow continues. Thus the SCR can only be commutated by an outside mechanism, which in effect reverse-biases the SCR. Although in principle this operation is relatively simple nonetheless commutation failures occasionally occur, and it is therefore necessary that an acceptable SCR control system somehow monitor and accommodate this contingency. While various systems have been devised to this end, they usually add considerable expense and complexity to the control circuit. Coupled with the requirement for recognizing commutation failure is the need to vary the current passed by the SCR to the battery charging circuit. This is ordinarily done by varying the point at which the SCR is gated on, thus varying the total conduction angle or on-time for each period. By varying the time and/or frequency at which the SCR is gated, the charging of a battery can be controlled.
In order that battery charging proceed properly, it is further necessary to monitor the charging current and battery voltage, and to feed back information regarding them to the SCR control circuit so that the gating of the SCR's can be varied in response to perceived current and voltage conditions. Where multiple SCR's are used and circuit resonance is relied upon for commutation, it is especially necessary that triggering of successive SCR's be accomplished only at such times as will not interfere with the commutation of the previously-gated thyristor.
It will be recognized that in order to accommodate all of the various constraints upon the operation of thyristor-controlled battery charger circuits, a functionally complex control circuit is necessary. At the same time, however, from a commercial point of view it is desirable to minimize the actual amount of circuitry both to minimize the size of a battery charger, and more importantly, its cost.
For all of the foregoing reasons, it will be recognized that it would be extremely advantageous to provide a control system for a thyristor-operated battery charger which achieves satisfactory voltage and current regulation, timing, and commutation failure recognition in a simplified and less expensive manner.
It is accordingly an object of the present invention to provide an improved battery charging system of the thyristor-operated type.
It is yet another object to provide an improved system for controlling thyristors in a battery charger system.
Another object is to provide a system for facilitating the clearing of a commutation failure in a resonant battery charging circuit including one or more thyristors.
A further object is to provide simplified control means for anticipating commutation failures in the thyristor circuit of a battery charger.